Environmental standards for particulate and total mercury emissions by coal-fired power plants, petroleum refineries, chemical refineries, incinerators, metallurgical operations, thermal treatment units, and other particulate- and mercury-emitting facilities are becoming increasingly more demanding. Emissions of particulate material are already strictly regulated for such facilities. New regulations are currently under development not only to amend existing regulations to reduce further permissible levels of total mercury emissions for such facilities but also to regulate total mercury emissions from a wide variety of other types of operations not presently subject to such regulations. As used herein, "total mercury emissions" refers to the concentration of mercury in the gas stream in whatever form (e.g., whether in elemental form or compounded with other elements).
Electrostatic precipitators and filters are two common particulate removal systems for removing particulate material from a gas stream. In electrostatic precipitators, electrodes impart a negative electrical charge to the particulate material. The charged particulate material migrates to positively charged collection plates where the material is collected. The collected particulate material is periodically removed from the collection plates for disposal.
Electrostatic precipitators have limitations in many applications. Electrostatic precipitators are unable to remove effectively particulate material having inadequate resistivity to retain an electrical charge. For particulate material that can retain an electrical charge, the particle collection efficiency of electrostatic precipitators has been found to decrease over time. It is believed that the decrease is primarily a result of electrical spark over from the collection plates to the electrodes.
As a result of the limitations of electrostatic precipitators in many applications, a number of operations use filtration rather than electrostatic precipitators to remove particulate material from gas streams. In filtration systems, a filter contains pores smaller than the size of the particulate material to collect the particulate material on the filter as a collected particulate material layer and yield a treated gas stream. The collected particulate material layer is typically removed from the filter by ceasing the flow of the gas stream through the filter and contacting the filter with a reverse flow of a gas. The reverse gas flow dislodges the collected particulate material layer from the filter, and the dislodged layer falls into a hopper for disposal. As can be appreciated, a design objective for such filters is to minimize the frequency of cleaning of the filter, thereby keeping the filter operational for as long as possible.
A variety of filter types can be used. For example, filters may be composed of various types of fabrics. More recently, a new type of filter has been proposed which combines more than 99% retention efficiency for 5 micron or larger particulate material with stability of the filter at high temperatures. The filter is composed of thermally stable materials, including ceramics, glass bonded ceramics, glasses, and sintered metals.
While the above-described particulate removal systems remove particulate material, neither electrostatic precipitators nor filters remove vaporous forms of mercury from the gas stream, requiring the systems to be modified for mercury removal. In particular in coal-fired power plants, a variety of sorbents are often introduced upstream of the filter to remove mercury from the flue gas. The sorbents are recovered by the filter as part of the collected particulate material layer. As used herein, "sorbent" refers to a substance that absorbs, adsorbs, or entraps another substance.
The efficiency of sorbents frequently utilized for mercury removal in facilities of the above-noted nature are often inadequate for meeting the increasing regulatory demands concerning total mercury emissions. Additionally, such sorbents typically do not remove all forms of mercury. Since certain types of fuels or raw materials produce only certain forms of mercury when consumed, it is therefore often necessary to base the selection of the specific sorbent to be used upon the chemical composition of the fuel or raw material to be consumed in the operation.
Additional limitations exist with respect to known techniques for using sorbents in facilities of the above-noted nature. Specifically, because the mercury-containing sorbents are recovered as part of the collected particulate material layer, the use of sorbents typically exchanges a contaminated waste gas problem for a contaminated solid waste problem. Generally, a substantial amount of sorbent is required to treat a gas stream containing dilute concentrations of mercury. The collected particulate material layer containing the sorbent can be difficult to store, transport and dispose of. Depending upon the application, the storage and disposal of mercury-containing solid waste is stringently regulated. Further, it is difficult to remove the mercury from the collected particulate material layer. In this regard, the removal of the mercury is generally impractical due to the dilute mercury concentrations in the collected particulate material layer and the volume of material in the layer. As a result, sorbents generally are not regenerable but are typically discarded after use due to the high costs to recover the sorbent from the layer and recycle the recovered sorbent.